The easy (ish) way is to cut down the front pipe you have to cut it as close as you can to the cat but still be able to get a decent seal on the supplied flanged sleave
you may also have to trim the sleave by a few mm too the reason being if you don't get the position of the cat right it knocks on the anitroll bar (there is a small flatened section on the std exhaust where it sits over the roll bar)